Lab08: Lighting & Shaders

Goals

  • Learn about vertex and fragment shaders.
  • Learn the basics of GLSL (shading language)
  • Learn how phong lighting is implemented in shaders.
  • Learn how the shaders interact with the Opengl program.
  • How are lights placed in the scene.
  • What is the difference between Gouraud vs Phong Shading (this is different from Phong lighting!)

Resources

  • Notes and readings :  Recall that a link to the uTubeSIGGRAPH workshop on OpenGL is posted under Resource Links on the home page.  See also Mike Bailey’s Graphics Shaders book (Ch. 3 and Ch. 5).  It is your responsibility to read and learn the material.
  • Angel (the text)
    – introduces shaders beginning on p. 85. In particular, the InitShader function is described.
    – lists the GLSL openGL functions starting on p. 695
  • Reference pages for GLSL

Part 1: Questions

Go over the material discussed in class and read the above references. You should be able to answer the following questions.

  • Where does each shader fit into the graphics pipeline?
  • What functions are typically done in a vertex shader? In a fragment shader?
  • At a minimum, what must the vertex shader do? At a minimum, what must the fragment shader do?
  • What functions are still done by the fixed pipeline and so do not need to be done in the shaders?
  • What are attribute variables and uniform variables? What does “in” and “out” mean?
  • What are some differences between GLSL and C or C++ ?

The Car

You are already familiar with the Phong lighting model that you implemented in your ray tracer. In this part of the lab, you are to understand the Phong lighting model as implemented in the shaders. There are two common ways of doing this:

  • Gouraud Shading – color calculations are done in the vertex shader and interpolated for the fragment shader.
  • Phong Shading – color calculations are done in the fragment shader. This gives much better results.  WHY?

The base color of the surface may be obtained in several different ways. We will discuss each of them:

  • Using the “colors” array that you created when you modeled the object.
  • Using a uniform variable (model_color) that is set when you are about to draw the object
  • In the shaders.

Start with the car navigation program you used from the last lab. Modify it so that it can handle lighting. Here are the changes you need to make:

  • On the fileshare, in Lab08-lightingShaders, please download the following:
    – Shaders folder contains the different shader files you need for this lab.
    – you will also see the file “CodeSnipptes.txt” that contains some of the code you need to bring into your main.cpp. We will go over the items in this file.
  • You need to make sure that the normals are calculated properly for your shapes. The Cube class has an error in the quad method – you need to add the line:
    vec4 norm = face_normals[n];
    at the beginning. The Cylinder class is ok.
  • The drawWheel method in the Car class has one of the disks facing in the wrong direction. You want the outward normal of each disk to face away from the center of the wheel. You will need to think about how to fix it.
  • In each of the shapes (Cube, Cylinder, Disk), you also need to update createVAO() so that the buffer loads the normals. CodeSnipptes.txt contains what this should look like for the Cube. Modify this method in the other shapes.
  • In Shapes.cpp, you no longer need to draw the wire frame objects because the lighting will make the shapes easy to see. In each of the draw methods in Shapes.cpp, comment out the wire part. CodeSnippets.txt shows what the drawCube method would look like.
  • In the display() method of main.cpp, you need to set the location of the light. To help you see where the light is, draw a cube at the location of the light.
  • Once you have calculated the position of the light as it should be in the shader, you need to set the shader’s corresponding light parameter:
    glUniform4fv( glGetUniformLocation(program, “LightPosition”), 1, l_position );
    where l_position is the value of the light position in eye coordinates (which is what the shader wants).
  • Try running the code, first with the simple shader that you used last week, and then with the Gouraud and Phong shaders.
  • Once you feel confident that the code runs and does what you expect, add code that animates the camera so that it rotates around the y axis of the world coordinate system. Also, you might want to add key presses that change the location of the light. This movement will enable you to really test to make sure that the lighting is working.
  • Finally, figure out how to modify the code so that the light moves as though it is attached to the camera.


Car with Gouraud Lighting


Car with Phong Lighting

No later than noon on Thurs, Week 9, demonstrate this part of the lab (no need to submit this part of the code via the fileshare).